Field of the Invention
The present invention relates to a conductive paste composition for use in forming and the like a membrane electrode of an electrostrictive element.
Description of the Related Art
It is known that when electrodes are connected to both front and back surfaces of a dielectric film formed of an elastomer and a voltage is applied, a compressive force is given to the dielectric film by Maxwell stress (piezoelectric adverse effect) from the interfacial polarization brought about by static electricity, and the dielectric film contracts in the thickness direction and expands in the lateral direction (the direction orthogonal to the thickness direction). In recent years, an electrostrictive element comprising a dielectric film and electrodes, which is driven by the aforementioned principle, is studied.
As the electrostrictive element, an electrostrictive element having a dielectric film formed of an elastomer, membrane electrodes arranged on both front and back surfaces of the inner side of the peripheral edge of the dielectric film and capable of expansion and contraction following the expansion and contraction of the dielectric film, a rim-type frame arranged on the peripheral edge of one surface of the dielectric film for retaining the dielectric film in an expanding state, and an electricity collector connected to the peripheral edge of the membrane electrodes has been proposed (refer to, for example, Patent Document 1: Japanese Patent Laid-Open No. 2003-174205).
When a positive or negative voltage is applied to each membrane electrode via the electricity collector, the dielectric film of the electrostrictive element contracts in the thickness direction and expands in the lateral direction, but the dielectric film is restricted in expanding toward the outer side and expands toward the inner side, and protrudes toward one surface side to thereby form nearly a mountain-like shape as a whole, since the peripheral edge of the aforementioned dielectric film is retained by the rim-type frame. And, the membrane electrode expands following the behavior of the dielectric film so as to expand and is changed in its shape to nearly a mountain-like shape.
After that, the shape of the expanded dielectric film is almost restored to the original shape by release of the application of voltage, and the expanded membrane electrode is almost restored to the original shape following the behavior of restoration of the dielectric film.
The membrane electrode for use in the electrostrictive element is required to be capable of expansion and contraction following the transformation of the dielectric film formed of an elastomer. In addition to the capabilities of expansion and contraction, the membrane electrode is also required to be small in variation of electrical resistance when expanded.
There is such a problem that when the membrane electrode is formed with a conductive paste such as a silver paste containing silver powder compounded in a binder resin, the formed membrane electrode is short of flexibility and cracks are generated when it is expanded in a large degree, as a result, electrical resistance conspicuously increases. There is also another problem such that the membrane electrode formed with the above conductive paste cannot follow expansion and contraction of the aforementioned dielectric film and hinders the movement of the dielectric film.
For solving the above problems, it is known to form the membrane electrode with a conductive paste obtained by dissolving an elastomer having a functional group capable of hydrogen bonding and a glass transition temperature (Tg) of −10° C. or less in a solvent, and adding a flaky or acicular first metal filler and a lumpy second metal filler to the above solution (refer to, for example, Patent Document 2: Japanese Patent No. 5486268).
However, the conductive paste described in Patent Document 2 has such a disadvantage that when a membrane electrode for use in an electrostrictive element is formed, it is sometimes difficult to simultaneously satisfy sufficient expansion and contraction property, and conductivity, further, a shape retaining property, a thin film property, and durability.
Accordingly, an object of the present invention is to solve such a disadvantage and provide a conductive paste composition that is capable of forming an electrode having sufficient expansion and contraction property, and conductivity as the membrane electrode for use in an electrostrictive element in a dried state as a thin film and that is also inexpensive.